The in vitro incubation of cells separated from the human body is difficult in many cases. For example, it is difficult to infinitely grow cells, such as lymphocytes, separated from the human body in vitro. Usually, even when these cells are incubated in a culture medium, they are died within from 2 weeks to several months. Some cancer cells or epithelial cells can be subcultured in vitro in a medium having some formulation. However, it is hard to obtain the same in an amount sufficient for investigation.
Further, in order to grow internal cells in vitro, it is necessary to exert some stimulus on cells. Thus, a variety of physiologically active substances have to be added to a culture medium. However, the selection of the appropriate substance is problematic, and it takes much labor.
In recent years, In order to solve these problems, a method has been proposed in which a carcinogenic substance or the like is added to desired cells or these cells are irradiated with ultraviolet rays or radiation for mutageniza ion (transformation).
Such a treatment can provide cell lines (mutant strains) capable of infinite growth.
Further, a method has been also proposed in which cancer genes are injected into desired cells by a gene transfer method to cause transformation, obtaining cells which are infinitely grown.
Still further, besides the method using the transformation, a method has been also known in which cells that produce antibodies or immunomodulation factors (lymphokines) in vivo are fused with cells to be infinitely grown to establish fused cells (hybridomas) producing antibodies or immunomodulation factors in vitro.
These conventional methods have, however, involved the following problems.
First, in the method using chemicals or the method using the irradiation with ultraviolet rays or the like, it takes much time, from several months to several years, to establish cell lines which are stable in the mutagenicity obtained and other cell functions. Further, it takes considerable time to put cell lines established to practical use. Further, the number of mutant strains (mutagenization efficiency) obtained relative to the number of cells treated is small. Thus, it is difficult to easily establish cell lines from objective cells.
Further, the method of obtaining cell lines through cell fusion is useful in a stable system of producing cell-derived substances such as monoclonal antibodies, lymphokine or the like. However, in the fused cell lines obtained by this method, the cell response is said to be decreased or lost against in various immuno-reactions which occur in vivo.
Accordingly, it has been quite difficult to use the in vivo cell interaction in a reproducible cell system in vitro even by these methods.